The present invention relates to a method for preventing polymer scale deposition on the reactor walls in the polymerization of an ethylenically unsaturated monomer or, more particularly, to a method for preventing polymer scale deposition on the reactor walls and other surfaces coming into contact with the monomer in the polymerization of an ethylenically unsaturated monomer in an aqueous polymerization medium.
As is known, several types of polymerization procedures are undertaken in the preparation of polymers of ethylenically unsaturated monomers by polymerizing a variety of monomers including suspension polymerization, emulsion polymerization, solution polymerization, vapor-phase polymerization and bulk polymerization. A serious problem common to all of these polymerization methods is that the reactor walls and other surfaces coming into contact with the monomer in the course of the polymerization reaction are covered with polymer scale deposited thereon to cause various disadvantages.
Namely, polymer scale deposition on the reactor walls and other surfaces in the course of the polymerization reaction not only directly influences the yield of the polymer product but also to decrease the efficiency of cooling through the walls to adversely influence the productivity of the polymerization reactor. Moreover, the polymer scale sometimes falls off the reactor walls to be intermixed with the polymer product to cause heavy decrease in the product quality. When the monomer is toxic to human body as is the case with vinyl chloride monomer, in addition, a very serious problem is unavoidable in respect of the workers' health because the polymer scale deposited on the reactor walls must be removed to prepare for the next run of the polymerization while a considerably large amount of the unreacted monomer is usually adsorbed in the polymer scale if not to mention the disadvantage due to the large consumption of labor and time.
Limiting the type of the polymerization process to those carried out in an aqueous polymerization medium, i.e. suspension polymerization and emulsion polymerization, various proposals and attempts have been made in the prior art to prevent or reduce the amount of polymer scale deposition on the reactor walls, of which the major current is to provide a scale-preventing coating to the surface of the reactor walls. A typical class of the coating materials used in the prior art includes polar organic compounds such as amine compounds, quinone compounds, aldehyde compounds and the like. Although this method of coating is effective in its own way, a problem involved in the method consists in the use of an organic solvent to prepare a coating solution containing the polar organic compound because organic solvents are generally toxic to human body and in most cases inflammable to cause a danager of fire or explosion. When a water-soluble polar organic compound is used in place of the organic-soluble ones, on the other hand, the preventing effect of polymer scale deposition is too low to provide a practical solution of the problem.
The above mentioned method of coating with a polar organic compound is indeed effective to some extent when the type of the polymerization in an aqueous polymerization medium is suspension polymerization and the effectiveness by the coating treatment can be lasting over a certain length of time while the method is little effective in emulsion polymerization or in suspension polymerization in which an emulsifying agent is used in combination with a suspending agent.
In connection with the material forming the polymerization reactor, stainless steel-made polymerization reactors are conventionally used for the polymerization or, in particular, suspension polymerization of vinyl chloride while polymerization of styrene or copolymerization of styrene and acrylonitrile is usually performed in a glass-lined polymerization reactor despite the expensiveness of glass-lined reactors in comparison with stainless steel-made reactors because the amount of polymer scale deposition on the reactor walls would be too large to industrially practice the polymerization when a stainless steel-made polymerization reactor is used. Glass-lined polymerization reactors are, however, disadvantageous because the heat transfer coefficient through a glass-lined wall is remakably small and glass-lined polymerization reactors of large capacity can hardly be manufactured due to the ready breaking and difficulty in fabrication.